1. Field of the Disclosure
The present disclosure is related to an electro-optical sensor system and method of use that analyzes images of a scene to identify the presence of specific coloration, such as skin tones, and more particularly, to a system and method for use at night and in other environments with minimal ambient lighting.
2. Description of Related Art
Systems are known for analyzing color video imagery for the purposing of detecting particular tones of color that are common to many varieties of human skin. Currently several imaging techniques for detection of human skin tones during the daytime are known. Such systems can only be used where ambient light from the sun and/or artificial light sources provides a full spectrum of light energy from which the skin tone target signatures can be detected. Some systems use image processing of images obtained from color camera systems. More recently, systems using infrared cameras have also been developed. To date, the best results have been obtained when both visible and infrared cameras are combined. Several exemplary prior art systems for identification of human skin tone in a captured image are discussed herein.
Color—RGB Only: Much work has been done to utilize commercial video camera equipment, which provides either Red Green Blue (RGB), luminance-chrominance-chrominance YCrCb, or other similar types of color space renderings of the visible spectrum to identify skin tones in a captured image. An exemplary RGB system is disclosed in U.S. Pat. No. 6,690,822 to Chen et al. An exemplary YCrCb system is disclosed in U.S. Pat. No. 7,426,296 to Lee et al. These systems and methods, ultimately all based on visible color spectrum, often result in “false alarms” wherein objects in the scene are mistakenly read as human skin.
Short Wave Infrared Only: Systems that use the Short Wave Infrared (SWIR) waveband alone are also know for detecting the presence of human skin tones. In particular, the technique generally requires two separate sub-wavebands, such as a first waveband of about 1.0 to 1.3 microns and a second waveband of about 1.4 to 1.7 microns. Alternatively, the near infrared band (NIR) between 0.9 to 1.0 microns can be used instead of the first SWIR waveband. An active medical device has been developed which takes advantage of this phenomena. See U.S. Pat. No. 7,446,316 to Kilgore et al.
Color—RGB and SWIR: Research pioneered by students at the Air Force Institute of Technology (AFIT) in Wright Patterson Air Force Base, Ohio, have discovered that the number of false alarms can be reduced if the scene is imaged and analyzed in both the visible (approx. 0.4 to 0.7 micron wavelength) and short-wave infrared (1.0-2.0 micron wavelength) spectra. See e.g., Brooks, Adam L., Improved Multispectral Skin Detection and its Applications to Search Space Reduction for Dismount Detection Based on Histograms of Oriented Gradients, Thesis, Air Force Institute of Technology, March 2010. However, the AFIT research is based on analysis of discrete wavelengths within those spectra bands using data from a hyperspectral system. A hyperspectral system is too large and cumbersome for portable use. See also Nunez, Abel, A Physical Model of Human Skin and Its Application for Search and Rescue, Dissertation, Air Force Institute of Technology, December 2009 and Peskosky, Keith, Design of a Monocular Multispectral Skin Detection, Melanin Estimation, and False Alarm Suppression System, Thesis, Air Force Institute of Technology, March 2010.
These prior art imaging systems do not address performing a skin tone detection function at night or in other low ambient light conditions. Thermal imaging cameras and detectors for detection of objects at night are also known in the prior art. Thermal detectors operate in the 3-12 micron spectral band. Most objects at earthly temperatures emit radiation in this region as a strongly correlated function of their temperature. Thermal imaging thus relies only on emissions based on the target temperature, and does not require external illumination from the sun or artificial lighting. In the thermal band, and typically at night, warm-blooded animals and humans tend to emit more radiation than the surrounding environment, including the ground, vegetation, and most man-made objects such as buildings and parked automobiles. A thermal imaging system is therefore capable of detecting warm bodies against such backgrounds. However, a thermal detector would also detect other warm-blooded animals for the same reasons. Accordingly, an operator who is specifically looking for human targets will experience “false alarms” from animals identified by a thermal imaging system.
In view of these shortcomings of existing imaging systems, a need exists for a portable image identification or processing system that detects the presence of specific warm objects at night or in low ambient light. The system should have the ability to operate over a wide range of lighting conditions and against a variety of backgrounds. When used to identify a human, the system should be capable of skin detection with a high accuracy over most ranges of human skin tones. The system should also be capable of real time operation (e.g., a frame rate in excess of approximately 15 Hz) and should have a low false alarm rate. The presently disclosed system is adapted to address these issues.